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2 Answers
2

Player A cannot target SoF with Naturalize, because it is currently in the graveyard. If they wanted to target the SoF, they need to do so when they had priority. Player A cannot prevent the SoF from being sacrificed for its ability anyway, since the ability exists independently of the source that created it. Seal of Fire says, "Sacrifice Seal of Fire: Seal of Fire deals 2 damage to target creature or player." It is an activate ability whose [Cost] is "Sacrifice Seal of Fire." If Player B paid the cost to activate SoF, it would no longer be on the battlefield by the time the Active Player received priority.

112.3b Activated abilities have a cost and an effect. They are written as “[Cost]: [Effect.] [Activation instructions (if any).]” A player may activate such an ability whenever he or she has priority. Doing so puts it on the stack, where it remains until it’s countered, it resolves, or it otherwise leaves the stack. See rule 602, “Activating Activated Abilities.”

116.1. Unless a spell or ability is instructing a player to take an action, which player can take actions at any given time is determined by a system of priority. The player with priority may cast spells, activate abilities, and take special actions.

112.7a Once activated or triggered, an ability exists on the stack independently of its source. Destruction or removal of the source after that time won’t affect the ability.

It is not possible to sacrifice a permanent twice. After you sacrifice a permanent, it is placed into its owner's graveyard.

701.14a To sacrifice a permanent, its controller moves it from the battlefield directly to its owner’s graveyard. A player can’t sacrifice something that isn’t a permanent, or something that’s a permanent he or she doesn’t control. Sacrificing a permanent doesn’t destroy it, so regeneration or other effects that replace destruction can’t affect this action.

I was under the impression that because the ability had not resolved, as it was interrupted by the naturalize, it was placed on the stack before the card reached the graveyard.
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StormchildMar 3 '12 at 19:29

4

@Stormchild You pay all costs at the time you activate an ability (i.e. put it on the stack). The actions you take as part of paying costs don't use the stack.
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Alex PMar 3 '12 at 20:14

To put Alex's comment another way, the card goes to the graveyard at the same time as the ability is first placed on the stack. There is no opportunity for you or your opponent to do anything between the ability being placed on the stack and the moving of the Seal to the graveyard. The casting of Naturalize happens well after both of those actions.
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David ZMar 4 '12 at 7:46

user1873 is right. Scenario 2 is not legal since the sacrificing is a cost. To try to it clarify further:
If Seal of Fire instead did "T: Destroy ~this~ and deal 2 damage to target player" it would still deal 2 damage to the player if you destroyed it before the ability resolved.

112.7a Once activated or triggered, an ability exists on the stack independently of its source. Destruction or removal of the source
after that time won't affect the ability. Note that some abilities
cause a source to do something (for example, "Prodigal Sorcerer deals
1 damage to target creature or player") rather than the ability doing
anything directly. In these cases, any activated or triggered ability
that references information about the source because the effect needs
to be divided checks that information when the ability is put onto the
stack. Otherwise, it will check that information when it resolves. In
both instances, if the source is no longer in the zone it's expected
to be in at that time, its last known information is used. The source
can still perform the action even though it no longer exists.

A common analogy is that of a soldier throwing a grenade at you. Shooting the soldier after he has thrown the grenade won't prevent the grenade from killing you

The only way you can stop it is either counter the ability (Stifle) or prevent it from fulfill other restrictions like destroying mountains for Valakut, the molten pinnacle's land limit.